A transmissive liquid crystal cell has a structure as shown in FIG. 8, where glass layers 31, 32 constitute the uppermost layer and the lowermost layer, respectively, and transparent electrode films 33, 34 are formed on the inner surfaces of the glass layers 31, 32, respectively. Furthermore, alignment films (not shown) are formed on the inner surfaces of the transparent electrode films 33, 34, which are each rubbed at a predetermined angle. A liquid crystal 15a is sealed in between the alignment films. In the case of a color liquid crystal cell, a color filter is disposed on the (inner surface of the) upper glass layer 31.
FIG. 9 illustrates the structure of a reflective liquid crystal cell. When compared with the transmissive liquid crystal cell, this is different from the transmissive liquid crystal cell in that a reflective metal film 35 is formed in lieu of the transparent electrode 34 on one side. An alignment film is formed on the surface of the reflective metal film 35, which is rubbed at a predetermined angle.
Suppose that the thickness (cell gap) of the liquid crystal contained in a liquid crystal cell is represented by d.
There has been a conventional method for measuring the cell gap d of a liquid crystal cell, in which light is directed from above the cell without liquid crystal injected therein so that interference of light waves is measured to obtain the thickness of the air layer, which is regarded as the thickness of the liquid crystal cell (Interferometry).
However, the thickness of the air layer before injection of the liquid crystal and that of the liquid crystal cell after injection of the liquid crystal are not identical in a strict sense. Accordingly, developing a method for directly measuring the thickness of a cell after injection of a liquid crystal has been anticipated.
Another known method is a method in which the birefringence of uniaxial crystals that liquid crystals have is utilized in such a manner that a linearly polarized component of light of a light source is introduced into a liquid crystal cell filled with a liquid crystal (hereinafter referred to as the “sample”), and the intensity of the transmitted light in a cross Nicol state and the intensity of the same in a parallel Nicol state are each measured. Based on these intensities, a birefringent phase difference (which is called “retardation”) of the liquid crystal is determined, from which the cell gap is obtained. (Refer to H. L. Ong, Appl. Phys. Lett. 51 (18), 2 Nov. 1987, pp 1398–1400, etc.)
Incidentally, in the foregoing method, since light is directed to one side of the sample and the light that has been transmitted through the sample is measured, the light source and the photodetector of the measurement system are opposed to each other with respect to the sample. For this reason, a problem arises in that the measurement system has to be large and cannot be compact in size.
In addition, since the foregoing method measures light transmitted through the sample, it is inapplicable, in principle, to reflective liquid crystal cells that do not allow light to pass through.
It is therefore an object of the present invention to provide a method for measuring gap of liquid crystal cell which is capable of measuring the gaps of liquid crystal cells and applicable to both transmissive and reflective liquid crystal cells, while allowing the size of the measurement system to be compact.